Stimulation and Inhibition by Normal Human Serum of Colony Formation in Vitro by Bone Marrow Cells

Summary. Normal human sera were able to stimulate granulocyte and macrophage colony formation in vitro by mouse bone marrow cells, but only following removal of inhibitory material either by dialysis or by heat‐ or ether‐treatment. Inhibitors were shown to be present in all normal human sera tested, to exhibit partial species specificity, to be multiple and to be probably lipoprotein in nature. The in vivo function of these inhibitors is unknown but they may modulate the action of the colony stimulating factor (CSF) in stimulating granulopoiesis and macrophage formation.